1. Field of the Invention
The present invention relates to a measuring device for the digital triggering of a test signal with superposed noise signals and can be used in a recording device, for example, in a digital oscilloscope.
2. Discussion of the Background
A phase-corrected or time-corrected display of a test signal on a recording device, for example, a digital oscilloscope, requires a triggering, which identifies the signal portion of the test signal to be displayed on the recording device, for example, via a trigger threshold value, and initiates the recording of the test signal on the recording device via a trigger signal derived from it at the trigger time.
However, interference which is superposed as noise on the signal to be measured can be problematic. As illustrated in FIG. 2, such interference can cause the triggering of an oscilloscope to be released in error. In FIG. 2, a trigger is to be released on a rising signal edge; however, the trigger is also released in the case of a falling edge of the signal to be measured, because interference with a rising edge is superposed on the signal to be measured at the level of the trigger threshold. With a continuous triggering, so-called shadows appear because, as a result of the error triggering events, the test signal to be displayed is not fixed in its position. These shadows have an interfering influence for the user who wishes to evaluate the test signal; or respectively, an automated evaluation of the test data is made significantly more difficult in the case of an error triggering (erroneously identified trigger event).
To avoid an error triggering of this kind, comparators with hysteresis must be used in the trigger system of an oscilloscope. In this context, the greater the background noise of the oscilloscope, the wider the hysteresis must be selected to be.
A triggering circuit, which reduces error triggering events through the use of comparators with hysteresis, is known from DE 10 2006 027 835 A1. As soon as the signal to be measured exceeds an upper hysteresis threshold, a timer is started. If the test signal does not fall below the lower hysteresis threshold within a fixed time span, a trigger pulse is output. If the lower hysteresis threshold is undercut before the time span of the timer has elapsed, no trigger signal is released.
The disadvantage with DE 10 2006 027 835 A1 is that the hysteresis thresholds cannot be adjusted arbitrarily so that test signals can still be safely triggered with different strengths of interference.